(1) Field of the Invention
This invention relates to electroluminescent display devices, particularly active matrix display devices having thin film switching transistors associated with each pixel.
(2) Description of Related Art
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer.
The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer. Ink-jet printing may also be used. Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays. Alternatively, these materials may be used for active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display element.
Display devices of this type have current-driven display elements, so that a conventional, analogue drive scheme involves supplying a controllable current to the display element. It is known to provide a current source transistor as part of the pixel configuration, with the gate voltage supplied to the current source transistor determining the current through the display element. A storage capacitor holds the gate voltage after the addressing phase.
FIG. 1 shows a known pixel circuit for an active matrix addressed electroluminescent display device. The display device comprises a panel having a row and column matrix array of regularly-spaced pixels, denoted by the blocks 1 and comprising electroluminescent display elements 2 together with associated switching means, located at the intersections between crossing sets of row (selection) and column (data) address conductors 4 and 6. Only a few pixels are shown in the Figure for simplicity. In practice there may be several hundred rows and columns of pixels. The pixels 1 are addressed via the sets of row and column address conductors by a peripheral drive circuit comprising a row, scanning, driver circuit 8 and a column, data, driver circuit 9 connected to the ends of the respective sets of conductors.
The electroluminescent display element 2 comprises an organic light emitting diode, represented here as a diode element (LED) and comprising a pair of electrodes between which one or more active layers of organic electroluminescent material is sandwiched. The display elements of the array are carried together with the associated active matrix circuitry on one side of an insulating support. Either the cathodes or the anodes of the display elements are formed of transparent conductive material. The support is of transparent material such as glass and the electrodes of the display elements 2 closest to the substrate may consist of a transparent conductive material such as ITO so that light generated by the electroluminescent layer is transmitted through these electrodes and the support so as to be visible to a viewer at the other side of the support. Typically, the thickness of the organic electroluminescent material layer is between 100 nm and 200 nm. Typical examples of suitable organic electroluminescent materials which can be used for the elements 2 are known and described in EP-A-0 717446. Conjugated polymer materials as described in WO96/36959 can also be used.
FIG. 2 shows in simplified schematic form a known pixel and drive circuitry arrangement for providing voltage-programmed operation. Each pixel 1 comprises the EL display element 2 and associated driver circuitry. The driver circuitry has an address transistor 16 which is turned on by a row address pulse on the row conductor 4. When the address transistor 16 is turned on, a voltage on the column conductor 6 can pass to the remainder of the pixel. In particular, the address transistor 16 supplies the column conductor voltage to a current source 20, which comprises a drive transistor 22 and a storage capacitor 24. The column voltage is provided to the gate of the drive transistor 22, and the gate is held at this voltage by the storage capacitor 24 even after the row address pulse has ended. The drive transistor 22 draws a current from the power supply line 26.
The drive transistor 22 in this circuit is implemented as a PMOS TFT, so that the storage capacitor 24 holds the gate-source voltage fixed. This results in a fixed source-drain current through the transistor, which therefore provides the desired current source operation of the pixel.
The above basic pixel circuit is a voltage-programmed pixel, and there are also current-programmed pixels which sample a drive current. However, all pixel configurations require current to be supplied to each pixel.
One problem with voltage-programmed pixels, particularly using polysilicon thin film transistors, is that different transistor characteristics across the substrate (particularly the threshold voltage) give rise to different relationships between the gate voltage and the source-drain current, and artefacts in the displayed image result.
It has been recognised that a current-programmed pixel can reduce or eliminate the effect of transistor variations across the substrate. For example, a current-programmed pixel can use a current mirror to sample the gate-source voltage on a sampling transistor through which the desired pixel drive current is driven. The sampled gate-source voltage is used to address the drive transistor. This partly mitigates the problem of uniformity of devices, as the sampling transistor and drive transistor are adjacent each other over the substrate and can be more accurately matched to each other. Another current sampling circuit uses the same transistor for the sampling and driving, so that no transistor matching is required, although additional transistors and address lines are required.
A further problem with LED displays arises from the significant currents drawn by the pixels. The displays are typically backward-emitting, through the substrate carrying the active matrix circuitry. This is the preferred arrangement because the desired cathode material of the EL display element is opaque, so that the emission is from the anode side of the EL diode, and furthermore it is not desirable to place this preferred cathode material against the active matrix circuitry. Metal row conductors are formed to define power supply lines, and for these backward emitting displays they need to occupy the space between display areas, as they are opaque. For example, in a 12.5 cm (diagonal) display, which is suitable for portable products, the row conductor may be approximately 11 cm long and 20 μm wide. For a typical metal sheet resistance of 0.2 Ω/square, this gives a line resistance for a metal row conductor of 1.1 kΩ. A bright pixel may draw around 8 μA and the current drawn is distributed along the row. The significant row conductor resistance gives rise to voltage drops along the row conductors, and these voltage variations along the power supply line alter the gate-source voltage on the drive transistors, and thereby affect the brightness of the display. Furthermore, as the currents drawn by the pixels in the row are image-dependent, it is difficult to correct the pixel drive levels by data correction techniques, and the distortion is essentially a cross talk between pixels in different columns.
The voltage drops can be reduced by a factor of 4 by drawing current from both ends of the row, and improvements in efficiency of the EL materials can also reduce the current drawn. Nevertheless significant voltage drops are still present. These voltage drops also give rise to performance limitations in current mirror pixel circuits, and thin film transistors are inherently non-ideal current source devices (the output current will in fact depend on both the source and drain voltages rather than only on the gate-source voltage).